Insulin resistance is a marker for increased risk of Non-Insulin-Dependent Diabetes Mellitus (NIDDM), Gestational Diabetes Mellitus (GDM), hypertension, and cardiovascular disease, and is a major cause of glucose intolerance in NIDDM, GDM, and obesity. Within the context of Syndrome X, insulin resistance is thought to be a primary lesion which leads to variable manifestations of glucose intolerance, hypertension, obesity, dyslipidemia, and premature atherosclerosis. While individuals without overt disease exhibit marked differences in insulin sensitivity/resistance, the degree to which relative insulin resistance is determined by genetic and environmental factors is unknown. The overall goal in the proposed studies is to define the heritability of insulin resistance by studying monozygotic (MZ) and dizygotic (DZ) twins.